The present invention relates to an electro-dynamic transducing device for producing ultrasonic waves with a unidirectional characteristics, and including two coils arranged in a lanbda/4 staggered (interdigitized) relationship and in the direction of beaming; these two coils are fed with high frequency currents so that acoustic signals result which interfer in a reinforcing fashion in the direction of beaming but cancel each other (or are supposed to cancel) in the opposite direction.
A transducer of the type to which the invention pertains is, for example, disclosed and described in German Pat. No. 2,655,804, particularly column 8, line 44 et seq. The transducer described a Lamb wave transducer but the present invention refers more broadly also to transducer which may use surface and plate mode osciallations, also tubular wave kind conductor transducers ought to be included.
Transducers generally of the type in which windings receive simultaneously a phase-shifted high frequency signal offers the advantage that, indeed, a large ultra-sonic amplitude is produced in the direction of beaming. However, this kind of device is disadvantaged by the fact that the feeding of a hgh frequency signal will produce initially a first acoustic wave by means of the rear coil without concurrent production of an acoustic wave by the front coil in that location of the rear coil so that the cancelling effect in the undersided direction does not obtain. As a consequence a certain parasitic wave is produced which, in turn, may result subsequently in undesired parasitic echo effects.